Genetic and phenotypic analysis of seizure susceptibility in PL/J mice

Epilepsy is one of the most common but genetically complex neurological disorders in humans. Identifying animal models that recapitulate human epilepsies is important for pharmacological studies of anticonvulsants, dissection of molecular and biochemical pathogenesis of epilepsy, and discovery of epilepsy susceptibility genes. We discovered that the PL/J inbred mouse strain is susceptible to handling- and rhythmic tossing–induced seizure. The tonic–clonic and generalized seizures observed after induction were accompanied by abnormal EEGs, similar to seizures observed in EL and SWXL-4 mice. PL/J mice also had an extremely low threshold to electroconvulsive seizures compared to other strains and showed variable sensitivity to pentylenetetrazole-induced seizures. Gross neurostructural abnormalities were not found in PL/J mice. Crosses with the seizure-resistant C57BL/6 J strain revealed semidominant inheritance of the rhythmic tossing seizure trait with low penetrance. F2 progeny indicated that the genetic inheritance of seizure susceptibility in PL/J is non-Mendelian. We crossed DBA/2 J mice, which are resistant to rhythmic tossing seizure but susceptible to audiogenic seizures, to PL/J. We found that seizure penetrance in (DBA/2 J × PL/J)F1 mice was similar to the penetrance in (C57BL/6 J × PL/J)F1 mice but the severity and frequency of seizure were higher in (DBA/2 J × PL/J)F1 mice. The PL/J strain serves as an interesting new model for studying the genetics, neurobiology, and pharmacology of epilepsy.     

Multiple seizure susceptibility genes on chromosome 7 in SWXL-4 congenic mouse strains

The SWXL-4 recombinant inbred mouse strain is unusually sensitive to recurrent tonic-clonic seizures upon routine handling and to seizures induced by chemoconvulsants. In a conventional intercross with the ABP/Le strain, we previously mapped a SWXL-4-derived quantitative trait locus called Szf1 (seizure frequency 1) to Chromosome 7. In the present study, we confirm the existence of Szf1 in both an independent cross and a congenic strain. However, derivative congenic recombinant strains show that an interaction between at least two genes on Chromosome 7-each of which has a very small effect on its own-account for Szf1.     

γ-Glutamyl Cyclotransferase: A New Genetic Polymorphism in the Mouse (MUS MUSCULUS) Linked to LYT–2

Electrophoretically variant forms of gamma-glutamyl cyclotransferase have been identified in red cells of inbred mouse strains. Each inbred strain exhibited a major band of activity and a minor band that migrated more anodally. The polymorphism affects the migration of both the major and minor bands in a similar way. F1 hybrids between strains with fast forms (A/J) and strains with the slow forms (C57BL/6J) exhibited a four-banded pattern consistent with co-dominant inheritance. The patterns observed in backcross and F2 mice were consistent with the segregation of a pair of autosomal co-dominant alleles. Recombinant inbred strains and a congenic strain were used to show that the locus controlling gamma-glutamyl cyclotransferase (Ggc) is linked to Lyt-2, a lymphocyte alloantigen locus on chromosome 6, with an estimated map distance of 5.0 +/- 2.5 centimorgans.     

Electroconvulsive Thresholds of Inbred Mouse Strains

The electroconvulsive threshold (ECT) test is used commonly in the screening of anti-epileptic drugs in rodent models, but little is known about its genetic or mechanistic basis. Thresholds for minimal clonic, maximal tonic, or psychomotor (partial) seizures were determined in 16 different inbred mouse strains in two different laboratories. A wide range of thresholds was observed, suggesting that a variety of neuroexcitability alleles exist in inbred strains. Although there was generally good cross-strain correlation between the three seizure types, several outlier strains were detected, showing that genetically encoded differences can affect the ability of a particular seizure type to spread through the brain. Furthermore, the relative seizure susceptibility of a strain was comparable between the two laboratories, suggesting that despite different test sites, instrumentation, and personnel, the ECT assay is portable and that common inbred strains can often be relied upon as calibration standards. Last, the ECT paradigm was also sensitive enough to detect single locus differences, laying the groundwork for mutation screens for new neuroexcitability models.     

Use of chromosome substitution strains to identify seizure susceptibility loci in mice

Seizure susceptibility varies among inbred mouse strains. Chromosome substitution strains (CSS), in which a single chromosome from one inbred strain (donor) has been transferred onto a second strain (host) by repeated backcrossing, may be used to identify quantitative trait loci (QTLs) that contribute to seizure susceptibility. QTLs for susceptibility to pilocarpine-induced seizures, a model of temporal lobe epilepsy, have not been reported, and CSS have not previously been used to localize seizure susceptibility genes. We report QTLs identified using a B6 (host) × A/J (donor) CSS panel to localize genes involved in susceptibility to pilocarpine-induced seizures. Three hundred fifty-five adult male CSS mice, 58 B6, and 39 A/J were tested for susceptibility to pilocarpine-induced seizures. Highest stage reached and latency to each stage were recorded for all mice. B6 mice were resistant to seizures and slower to reach stages compared to A/J mice. The CSS for Chromosomes 10 and 18 progressed to the most severe stages, diverging dramatically from the B6 phenotype. Latencies to stages were also significantly shorter for CSS10 and CSS18 mice. CSS mapping suggests seizure susceptibility loci on mouse Chromosomes 10 and 18. This approach provides a framework for identifying potentially novel homologous candidate genes for human temporal lobe epilepsy.     

A locus for radiation-induced gastroschisis on mouse Chromosome 7

Gastroschisis (abdominal wall defects) occurs with a high frequency in the mouse inbred strain HLG compared with C57BL/6J mice. The risk of gastroschisis increases significantly after exposure to irradiation with X-rays during preimplantation development and follows a recessive mode of inheritance for the HLG susceptibility alleles. We have used a backcross strategy and genome-wide microsatellite typing to chromosomally map this trait. A suggestive linkage for a locus responsible for radiation-induced gastroschisis (Rigs1) was found in a region of mouse Chromosome 7. Received: 13 May 1998 / Accepted: 6 August 1998     

Assignment of the structural gene for argininosuccinate synthetase to proximal mouse chromosome 2

In order to develop linkage markers for the murine argininosuccinate synthetase locus (Ass-1), we have searched for restriction fragment length polymorphisms in the mouse genome using cloned sequences from the mouse arginosuccinate synthetase structural gene. Five restriction fragment length polymorphisms were found among the recombinant inbred progenitor strains AKR/J, BALB/cByJ, C3H/HeJ, C57BL/6J, C57L/J, DBA/2J, and SWR/J. Of these, four polymorphisms were found to distinguish the SWR/J strain from the other six strains, which all had the same fragment. The fifth polymorphism revealed differences among the progenitor strains for recombinant inbred strain sets AKXL, BXD, and SWXL. The strain distribution pattern for this polymorphism indicated close linkage of Ass-1 to Hc (the fifth component of complement) on proximal mouse chromosome 2 with a recombination fraction of 0.016 and a 95% confidence interval of 0.003 to 0.054. These data place Ass-1 in a syntenic group with the genes Hc, Abl, Fpgs, and Ak-1 whose linkage has been conserved between human chromosome 9q and mouse chromosome 2.     

Phenotyping mouse chromosome substitution strains reveal multiple QTLs for febrile seizure susceptibility

Febrile seizures (FS) are the most common seizure type in children and recurrent FS are a risk factor for developing temporal lobe epilepsy. Although the mechanisms underlying FS are largely unknown, recent family, twin and animal studies indicate that genetics are important in FS susceptibility. Here, a forward genetic strategy was used employing mouse chromosome substitution strains (CSS) to identify novel FS susceptibility quantitative trait loci (QTLs). FS were induced by exposure to warm air at postnatal day 14. Video electroencephalogram monitoring identified tonic–clonic convulsion onset, defined as febrile seizure latency (FSL), as a reliable phenotypic parameter to determine FS susceptibility. FSL was determined in both sexes of the host strain (C57BL/6J), the donor strain (A/J) and CSS. C57BL/6J mice were more susceptible to FS than A/J mice. Phenotypic screening of the CSS panel identified six strains (CSS1, -2, -6 -10, -13 and -X) carrying QTLs for FS susceptibility. CSS1, -10 and -13 were less susceptible (protective QTLs), whereas CSS2, -6 and -X were more susceptible (susceptibility QTLs) to FS than the C57BL/6J strain. Our data show that mouse FS susceptibility is determined by complex genetics, which is distinct from that for chemically induced seizures. This is the first data set using CSS to screen for a seizure trait in mouse pups. It provides evidence for common FS susceptibility QTLs that serve as starting points to fine map FS susceptibility QTLs and to identify FS susceptibility genes. This will increase our understanding of human FS, working toward the identification of new therapeutic targets.     

A Parent-of-Origin Effect Determines the Susceptibility of a Non-Informative F1 Population to Trypanosoma cruzi Infection In Vivo

The development of Chagas disease is determined by a complex interaction between the genetic traits of both the protozoan parasite, T. cruzi, and the infected host. This process is regulated by multiple genes that control different aspects of the host-parasite interaction. While determination of the relevant genes in humans is extremely difficult, it is feasible to use inbred mouse strains to determine the genes and loci responsible for host resistance to infection. In this study, we investigated the susceptibility of several inbred mouse strains to infection with the highly virulent Y strain of T. cruzi and found a considerable difference in susceptibility between A/J and C57BL/6 mice. We explored the differences between these two mouse strains and found that the A/J strain presented higher mortality, exacerbated and uncontrolled parasitemia and distinct histopathology in the target organs, which were associated with a higher parasite burden and more extensive tissue lesions. We then employed a genetic approach to assess the pattern of inheritance of the resistance phenotype in an F1 population and detected a strong parent-of-origin effect determining the susceptibility of the F1 male mice. This effect is unlikely to result from imprinted genes because the inheritance of this susceptibility was affected by the direction of the parental crossing. Collectively, our genetic approach of using the F1 population suggests that genes contained in the murine chromosome X contribute to the natural resistance against T. cruzi infection. Future linkage studies may reveal the locus and genes participating on the host resistance process reported herein.     

Pas1 is a common lung cancer susceptibility locus in three mouse strains

Inherited predisposition to lung cancer is a phenotypic trait shared by different mouse inbred strains that show either a high or an intermediate predisposition. Other strains are instead genetically resistant. The Pas1 locus is the major determinant of lung cancer predisposition in the A/J strain (Gariboldi et al. 1993). To define the determinants of susceptibility to lung tumorigenesis in the highly susceptible SWR/J and in the intermediately susceptible BALB/c mice, we analyzed (BALB/c × SWR/J)F₂ and (BALB/c × C3H/He)F₂ crosses by genetic linkage experiments. The present results provide unequivocal evidence that the same Pas1/+ allele that leads to lung cancer predisposition is shared by A/J, SWR/J, and BALB/c strains. The intermediate susceptibility of the BALB/c strain would result by interaction of Pas1 locus with lung cancer resistance loci. Received: 18 April 1997 / Accepted: 15 June 1997     

Genetic complexity of absence seizures in substrains of C3H mice

Absence epilepsy is a common form of idiopathic generalized epilepsy whose etiology is poorly understood because of genetic and phenotypic heterogeneity. The inbred mouse strain C3H/He exhibits spontaneous absence seizures characterized by spike and wave discharges (SWD) on the electroencephalogram concomitant with behavioral arrest. Previous studies using the C3H/HeJ (HeJ) substrain identified a mutation in the Gria4 gene as a major susceptibility locus. In the present study, we found that two closely related substrains C3H/HeOuJ (OuJ) and C3H/HeSnJ, which have a similar SWD incidence as HeJ, do not contain the Gria4 mutation. Further analysis of backcross mice segregating OuJ and C57BL/6J alleles shows that, unlike the HeJ substrain, OuJ does not have a major locus for SWD but has suggestive loci at best that would explain only a fraction of the phenotypic variance. These results illustrate how the genetic etiology of a common neurological disorder can differ between substrains with similar phenotypes. We infer that all C3H strains are sensitized to SWD and that additional mutations affecting SWD arose or were fixed independently in the years since the substrains diverged.     

Transmission-Ratio Distortion through F(1) Females at Chromosome 11 Loci Linked to Om in the Mouse Ddk Syndrome

We determined the genotypes of >200 offspring that are survivors of matings between female reciprocal F(1) hybrids (between the DDK and C57BL/6J inbred mouse strains) and C57BL/6J males at markers linked to the Ovum mutant (Om) locus on chromosome 11. In contrast to the expectations of our previous genetic model to explain the ``DDK syndrome,' the genotypes of these offspring do not reflect preferential survival of individuals that receive C57BL/6J alleles from the F(1) females in the region of chromosome 11 to which the Om locus has been mapped. In fact, we observe significant transmission-ratio distortion in favor of DDK alleles in this region. These results are also in contrast to the expectations of Wakasugi's genetic model for the inheritance of Om, in which he proposed equal transmission of DDK and non-DDK alleles from F(1) females. We propose that the results of these experiments may be explained by reduced expression of the maternal DDK Om allele or expression of the maternal DDK Om allele in only a portion of the ova of F(1) females.     

Segregation patterns of endogenous mouse mammary tumor viruses in five recombinant inbred strain sets.

We identified mouse mammary tumor proviral loci in the AKR/J, C3H/HeJ, C57BL/6J, C57L/J, DBA/2J, and SWR/J inbred mouse strains and determined their segregation patterns in the AKXD, AKXL, BXD, BXH, and SWXL recombinant inbred strain sets. Two new Mtv loci, Mtv-29 and Mtv-30, were identified. Mtv-30 was genetically mapped to chromosome 12. Additionally, two previously identified Mtv loci, Mtv-14 and Mtv-23, were genetically mapped to chromosome 4 and chromosome 6, respectively.     

Fine map of the Gct1 spontaneous ovarian granulosa cell tumor locus

The spontaneous development of juvenile-onset, ovarian granulosa cell (GC) tumors in the SWR/Bm (SWR) inbred mouse strain is a model for juvenile-type GC tumors that appear in infants and young girls. GC tumor susceptibility is supported by multiple Granulosa cell tumor (Gct) loci, but the Gct1 locus on Chr 4 derived from SWR strain background is fundamental for GC tumor development and uniquely responsive to the androgenic precursor dehydroepiandrosterone (DHEA). To resolve the location of Gct1 independently from other susceptibility loci, Gct1 was isolated in a congenic strain that replaces the distal segment of Chr 4 in SWR mice with a 47 × 10⁶-bp genomic segment from the Castaneus/Ei (CAST) strain. SWR females homozygous for the CAST donor segment were confirmed to be resistant to DHEA- and testosterone-induced GC tumorigenesis, indicating successful exchange of CAST alleles (Gct1 ^CA ) for SWR alleles (Gct1 ^SW ) at this tumor susceptibility locus. A series of nested, overlapping, congenic sublines was created to fine-map Gct1 based on GC tumor susceptibility under the influence of pubertal DHEA treatment. Twelve informative lines have resolved the Gct1 locus to a 1.31 × 10⁶-bp interval on mouse Chr 4, a region orthologous to human Chr 1p36.22.     

A DNA polymorphism from five inbred strains of the mouse identifies a functional class of domesticus-type Y chromosome that produces the same phenotypic distribution of gonadal hermaphrodites

The wild-derived CLA inbred strain of the house mouse contains a domesticus-type Y chromosome that lacks a 2.3-kb TaqI band with fragment 1 of the AC11 probe. The CLA Y chromosome also causes a low frequency of XY gonadal hermaphrodites when backcrossed to the C57BL/6J strain (F. G. Biddle and Y. Nishioka, 1988. Genome, 30:870-878). A similar domesticus-type Y chromosome, lacking the 2.3-kb TaqI band has now been found in the four historical inbred strains AKR/J, MA/MyJ, PL/J, and RF/J. When backcrossed to C57BL/6J, these four Y chromosomes cause low frequencies of gonadal hermaphrodites similar to the CLA Y and phenotypic distribution of types of gonad are indistinguishable from that with the CLA Y. The absence of the 2.3-kb TaqI band appears to be a polymorphism among domesticus-type Y chromosomes that identifies one of the three functional classes that, so far, can be distinguished only by their effects on testis differentiation in backcross test fetuses with the C57BL/6J strain. Three other historical inbred strains, BUB/BnJ, ST/bJ, and SWR/J, with a domesticus-type Y chromosome but containing the 2.3-kb TaqI band, were also assayed. They permit normal testis development in backcross test fetuses with C57BL/6J.     

Allelic profile at 37 biochemical loci of two inbred strains of the house mouse derived from wild Mus musculus musculus

Two newly established inbred strains derived from Mus musculus musculus, designated PWD/Ph (F29) and PWK/Ph (F33), were examined for their alleles at 37 biochemical loci located on 12 different chromosomes. The allelic pattern showed characteristic differences from those observed in common inbred strains. The genetic distance D between PWK/Ph and PWD/Ph was 0.027, whereas the corresponding values for the genetic distances between PWK/Ph and C57BL/6J, DBA/2J, BALB/cJ and SWR/J were 0.777, 0.721, 0.721 and 0.838 respectively. New allozymes are described as being controlled by the loci Es-23, Pre-2 and Tam-1. The genetic relationship to M.m.molossinus is indicated by identical alleles at six other loci (Es-2, Es-9, Es-10, Es-11, Es-18 and Es-22).     

Sex-restricted non-Mendelian inheritance of mouse Chromosome 11 in the offspring of crosses between C57BL/6J and (C57BL/6J × DBA/2J)F1 mice

We report on the observation of sex-restricted, non-Mendelian inheritance over a region of mouse Chromosome (Chr) 11, occurring in the offspring of crosses between two commonly used Mus musculus-derived inbred strains, C57BL/6J and DBA/2J. In the surviving backcross progeny of reciprocal matings between (C57BL/6J × DBA/2J)F₁ hybrids and the C57BL/6J parental strain, we observed the preferential appearance of C57BL/6J alleles along a region of Chr 11. The deviation from Mendelian predictions was observed only in female offspring from both reciprocal backcrosses, and not in males from either cross. The sex-specificity of the observed non-Mendelian inheritance points to an explanation based on embryonic or neonatal lethality. Our data add to previously obtained evidence for a Chr 11 locus or loci with sex-specific and allele-specific effects on viability. Received: 19 December 1997 / Accepted: 10 June 1998     

A new erythrocytic antigen of C57BL/10 (B10) mice

A new antigen, detectable on murine erythrocytes by hemagglutination assay with a (BALB/cCrl X SWR/J)F1 anti-B10.D2n/Sn alloantiserum, is described. Among the inbred and congenic mouse strains tested for reactivity with the antiserum, only the immunizing strain, B10.D2, and its congenic resistant partner, C57BL/10 (B10), reacted. Three other C57 strains, C57BL/6J, C57BL/6By, and C57L, were negative for the antigen. F1 hybrids between B10 and BALB/c, an antigen-negative strain, were positive for the antigen indicating that its expression is dominant. Typing of 39 (BALB/c X (BALB/c X B10)F1) and 62 [BALB/c X B10)F1 X BALB/c) backcross mice revealed that a single gene controls expression of the antigen. The gene is autosomal and not linked to H-2, Ly-4, or the c (albino) or b coat color genes.     

A major gene affecting age-related hearing loss is common to at least ten inbred strains of mice

Inbred strains of mice offer promising models for understanding the genetic basis of human presbycusis or age-related hearing loss (AHL). We previously mapped a major gene affecting AHL in C57BL/6J mice. Here, we show that the same Chromosome 10 gene (Ahl) is a major contributor to AHL in nine other inbred mouse strains-129P1/ReJ, A/J, BALB/cByJ, BUB/BnJ, C57BR/cdJ, DBA/2J, NOD/LtJ, SKH2/J, and STOCK760. F1 hybrids between each of these inbred strains and the normal-hearing inbred strain CAST/Ei retain good hearing, indicating that inheritance of AHL is recessive. To follow segregation of hearing loss, F1 hybrids were backcrossed to the parental strains with AHL. Auditory-evoked brain-stem response thresholds were used to assess hearing in more than 1500 N2 mice and analyzed as quantitative traits for linkage associations with Chromosome 10 markers. Highly significant linkage was found in all nine strain backcrosses, with the highest probability (LOD > 70) near the marker D10Mit112. This map position for Ahl is near the waltzer mutation (v) and the modifier of deaf waddler locus (mdfw), suggesting the possibility of allelism. Results from an intercross of C57BL/6J and NOD/LtJ mice indicate that the 6- to 10-month difference in AHL onset between these two strains is not due to allelic heterogeneity of the Ahl gene.     

The response of recombinant inbred strains of mice to bacterial lipopolysaccharides.

Fourteen recombinant inbred strains of mice have been produced by the inbreeding of the F2 generation of a cross between C57BL/6J and C3H/HeJ progenitor mice. The responses of these BXH strains to bacterial lipopolysaccharides (LPS) have been characterized. Four BXH strains are high LPS responders and nine strains are low LPS responders. One BXH strain shows intermediate responsiveness which may reflect residual heterozygosity. F1 hybrid mice from low x high responder strains were intermediate in their response to LPS suggesting additive genetic control. The LPS responses in backcross mice from the F1 x low LPS responders showed segregation consistent with LPS responsiveness being determined by a single gene. In 13/14 BXH strains, there was concordant inheritance of LPS responsiveness and the major urinary protein locus Mup-1b. The association of the expression of the Mup-1 alleles with LPS responsiveness in the BXH strains suggests that the defective LPS response gene in C3H/HeJ mice is located on chromosome 4.